Circuit for synchronizing the ignition of electronic ballast discharge lamps

ABSTRACT

A circuit for synchronizing the ignition in electronic ballast discharge lamps, wherein a half-bridge output stage of the electronic ballast has an upper switch and a lower switch. The lower switch is connected to a common wire of a power supply, and a high voltage impulse for igniting the lamp is generated only when the lower switch is open. The circuit includes a transformer including a first winding and a second winding magnetically coupled to each other, with the second winding having a lower number of turns than the first winding. One lead of the first winding is connectable to an electrode of a discharge lamp, and a second lead thereof is connectable to the common wire of the power supply. A diode is connected with its anode to one lead of the second winding, and with its cathode to the junction between the upper and lower switches. A second lead of the second winding is connected to a thyristor, which is connected to a first plate of a discharge capacitor, and a second plate of the capacitor is connected to the common wire, whereby a discharge current of the capacitor flows through the second winding only when the lower switch is open.

TECHNICAL FIELD

The present invention relates to the art of igniting discharge lamps,and more particularly, to a circuit for synchronizing the ignition ofelectronic ballast discharge lamps.

The invention is especially useful for igniting high intensity discharge(HID) lamps, e.g., high pressure sodium (HPS) lamps, while protectingthe ballast circuits against damage when the lamp does not ignite andrepeated attempts to do so are performed.

BACKGROUND ART

A great number of electronic ballast circuits for igniting HID lamps areknown and utilized. All known half-bridge ballasts suffer from thedrawback of their failure to protect the ballast against damage duringthe starting or striking period of the lamp when the lamp is out oforder and will not ignite.

For the sake of completeness and better understanding, a typical priorart HPS ballast circuit such as those referred to above, is shown inFIG. 1. The circuit is illustrated and described in detail in an articleby Peter N. Wood, entitled “Electronic Ballasts Using the Cost-SavingIR2155 Driver,” MOS-Gate Driver Data Book, E50161, InternationalRectifier Application Notes (1995).

DISCLOSURE OF THE INVENTION

It is therefore a broad object of the present invention to provide anelectronic ballast circuit for HID lamps which is capable of protectingthe ballast against damage during attempts to ignite it, when the lampis not susceptible to striking.

Therefore, in accordance with the present invention, there is provided acircuit for synchronizing the ignition in electronic ballast dischargelamps, including a half-bridge output stage of the electronic ballasthaving a upper switch and a lower switch, said lower switch beingconnected to a common wire of a power supply, in which circuit a highvoltage impulse for igniting the lamp is only generated when the lowerswitch is open, said circuit comprising a transformer including a firstwinding and a second winding magnetically coupled to each other, saidsecond winding having a lower number of turns than said first winding;one lead of said first winding being connectable to an electrode of adischarge lamp and a second lead thereof being connectable to the commonwire of said power supply; a diode connected with its anode to one leadof said second winding and with its cathode to the junction between saidupper and lower switches; a second lead of said second winding connectedto a symmetrical diode thyristor, said thyristor being connected to thefirst plate of a discharge capacitor and the second plate of saidcapacitor being connected to said common wire; whereby discharge currentof said capacitor flows through said second winding only when said lowerswitch is open.

The invention further provides a circuit for synchronizing the ignitionin electronic ballast discharge lamps, including a half-bridge outputstage of the electronic ballast having a upper switch and a lowerswitch, said lower switch being connected to a common wire of a powersupply, in which circuit a high voltage impulse for igniting the lamp isonly generated when the lower switch is open, said circuit comprising atransformer including a first winding and a second winding magneticallycoupled to each other, said second winding having a lower number ofturns than said first winding; one lead of said first winding beingconnectable to an electrode of a discharge lamp and a second leadthereof being connected to the junction between said upper and lowerswitches through a separating capacitor; a diode connected with itsanode to one lead of said second winding through a thyristor, and withits cathode to the junction between said first and second powerswitches; a second lead of said second winding being connected to saidcommon wire through a discharge capacitor; whereby discharge current ofsaid capacitor flows through said second winding only when said lowerswitch is open.

The invention still further provides a circuit for synchronizing theignition in electronic ballast discharge lamps, including a half-bridgeoutput stage of the electronic ballast having a upper switch and a lowerswitch, said lower switch being connected to a common wire of a powersupply, in which circuit a high voltage impulse for igniting the lamp isonly generated when the lower switch is open, said circuit comprising atransformer including a first winding and a second winding magneticallycoupled to each other, said second winding having a lower number ofturns than said first winding; one lead of said first winding beingconnectable to an electrode of a discharge lamp and a second leadthereof being connectable to the common wire of said power supply; adiode connected with its anode to one lead of said second windingthrough a thyristor, and with its cathode to the junction between saidfirst and second power switches; a second lead of said second windingbeing connected to said common wire through a discharge capacitor;whereby discharge current of said capacitor flows through said secondwinding only when said lower switch is open.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in connection with certain preferredembodiments with reference to the following illustrative figures so thatit may be more fully understood.

With specific reference now to the figures in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention, the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice.

In the drawings:

FIG. 1 is a circuit diagram of a prior art electronic ballast circuitfor an HPS lamp;

FIG. 2 is a diagram of a first embodiment of a synchronization circuitfor striking an

HID lamp according to the present invention;

FIG. 3 is a circuit diagram of the present invention as shown in FIG. 2,integrated into the prior art circuit of FIG. 1;

FIG. 4 is a further embodiment of the synchronization circuit accordingto the present invention; and

FIG. 5 is a modification of the circuit of FIG. 4.

DETAILED DESCRIPTION

The circuit of the prior art electronic ballast shown in FIG. 1 has twopower switches 2, 4 in a totem pole (half-bridge) configuration. Theseswitches 2 and 4 are power MOSFETs alternatively switched to theirconductive state by a MOS gate driver 6 (MGD IR 2155). The MGD 6provides a high-frequency square-wave signal with a frequency from 20 to80 kHz. As described in the above-mentioned article, the circuitprovides an input power factor of at least 0.9 and has DC bus controllimiting the voltage to 225 VDC, whether or not the lamp is energized.The transformer 10 performs two functions: (1) it limits the current forthe negative resistance characteristics of the lamp, and (2) it producesa pulse voltage step-up function to strike the HPS lamp 12. The 3 kVpulse voltage is derived from a 135 V symmetrical diode thyristor 14,e.g., a SIDAC or a DIAC, which discharges a 1 μf capacitor 16 into the2-turn winding of transformer 10. The 30:1 step ratio of transformer 10supplies the starting pulse to lamp 12. After the lamp strikes, there isinsufficient charge voltage on the 1 μf capacitor 16 in the 2-turnwinding circuit to avalanche the thyristor 14, and no further startpulses are supplied. The hot re-strike time of this ballast isapproximately 75 seconds.

Therefore, an attempt to re-strike a lamp with the ballast circuit ofFIG. 1, prior to expiration of the 75-second re-strike time when thelamp is not insusceptible to striking and the switch 4 is closed, willresult in the breakdown of the switch 4 and failure of the device as awhole. This is due to the fact that the resistance of the closed switch4 substantially exceeds the resistance of hot lamp 12. When ahigh-voltage striking pulse is supplied, the voltage drop across theswitch 4 substantially exceeds its operational voltage and results inits breakdown.

In the shown circuit, the supply of the high-voltage striking pulse isnot coordinated at all with the generation of the high-frequency squarewave signal, i.e., striking of the lamp 12 is not synchronized with theoperation of the output electronic stage of the ballast; therefore,striking the lamp in the period when switch 4 is closed, will result inballast failure.

Since an attempt to strike the arc lamp with the switch 4 closed isprobable when re-striking the lamp within a hot re-striking time period,e.g., within 75 seconds, the prior art ballast circuit of FIG. 1 canhardly be considered as providing striking of a lamp at the necessary,reliable level.

In order to overcome the above drawback, the invention provides acircuit for synchronizing the lamp's striking instance with theoperation of the output stage of the electronic ballast circuit. Such asynchronization circuit is shown in FIG. 2., and consists of transformer18 with two galvanically uncoupled windings E₁ and E₂, having differentnumbers of turns. The winding E₂ of transformer 18 has a lesser numberof turns, and is connected to the anode of diode 20. The cathode of saiddiode 20 is connected to junction 22 of the half-bridge output stage ofthe ballast between the upper and lower switches 2, 4 respectively. Thelower switch 4 is connected to the common wire 24. One plate ofcapacitor 16 is connected to the common wire 24, and the second plate ofcapacitor 16 is connected to thyristor 14 The other end of thyristor 14is connected to the second end of winding E₂ of transformer 18. Thewinding E₁ of transformer 18 is connected in series to one electrode ofthe discharge lamp 12.

In FIG. 3, there is shown the circuit of FIG. 1, modified to incorporatethe synchronizing circuit of FIG. 2.

As is clearly seen in FIG. 3, the synchronizing circuit assures that thedischarge current of capacitor 16 flows through winding E₂ oftransformer 18 only when lower switch 4 of the electronic ballast outputstage is open.

The same results may be achieved by connecting the various elements-in adifferent sequence, as shown in FIG. 4. One end of winding E₂ oftransformer 18 is connected to thyristor 14, which in turn is connectedto diode 20, leading to junction 22 between switches 2 and 4 of theoutput stage. The other end of the winding E₂ is connected to the commonwire 24 through the discharge capacitor 16.

FIG. 5 illustrates another possible embodiment of the present inventionin which one lead of the winding El is connected to the power supplywire 26 via resistor 28 and the second lead is effectively connected tothe common wire 24 through a separating capacitor 30 and the lowerswitch 4.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. A circuit for synchronizing the ignition of anelectronic ballast discharge lamp, including a half-bridge output stageof the electronic ballast having a upper switch and a lower switch, saidlower switch being connected to a common wire of a power supply, inwhich circuit a high voltage impulse for igniting the lamp is onlygenerated when the lower switch is open, said circuit comprising: atransformer including a first winding and a second winding magneticallycoupled to each other, said second winding having a lower number ofturns than said first winding; one lead of said first winding beingconnectable to an electrode of the discharge lamp and a second leadthereof being connectable to the common wire of said power supply; adiode connected with its anode to one lead of said second winding andwith its cathode to the junction between said upper and lower switches;a second lead of said second winding connected to a thyristor, saidthyristor being connected to the first plate of a discharge capacitorand the second plate of said capacitor being connected to said commonwire; whereby discharge current of said capacitor flows through saidsecond winding only when said lower switch is open.
 2. A circuit forsynchronizing the ignition of an electronic ballast discharge lamp,including a half-bridge output stage of the electronic ballast having aupper switch and a lower switch, said lower switch being connected to acommon wire of a power supply, in which circuit a high voltage impulsefor igniting the lamp is only generated when the lower switch is open,said circuit comprising: a transformer including a first winding and asecond winding magnetically coupled to each other, said second windinghaving a lower number of turns than said first winding; one lead of saidfirst winding being connectable to an electrode of the discharge lampand a second lead thereof being connected to the junction between saidupper and lower switches through a separating capacitor; a diodeconnected with its anode to one lead of said second winding through athyristor, and with its cathode to the junction between said upper andlower power switches; a second lead of said second winding beingconnected to said common wire through a discharge capacitor; wherebydischarge current of said capacitor flows through said second windingonly when said lower switch is open.
 3. A circuit for synchronizing theignition of an electronic ballast discharge lamp, including ahalf-bridge output stage of the electronic ballast having a upper switchand a lower switch, said lower switch being connected to a common wireof a power supply, in which circuit a high voltage impulse for ignitingthe lamp is only generated when the lower switch is open, said circuitcomprising: a transformer including a first winding and a second windingmagnetically coupled to each other, said second winding having a lowernumber of turns than said first winding; one lead of said first windingbeing connectable to an electrode of the discharge lamp and a secondlead thereof being connectable to the common wire of said power supply;a diode connected with its anode to one lead of said second windingthrough a thyristor, and with its cathode to the junction between saidfirst and second power switches; a second lead of said second windingbeing connected to said common wire through a discharge capacitor;whereby discharge current of said capacitor flows through said secondwinding only when said lower switch is open.
 4. The circuit as claimedin any one of the preceding claims, wherein said thyristor is asymmetrical diode thyristor.
 5. The circuit as claimed in claim 4,wherein said symmetrical diode thyristor is a SIDAC or a DIAC.
 6. Thecircuit as claimed in any one of claims 1, 2 or 3, wherein said upperand lower switches are MOSFETs.
 7. The circuit as claimed in claim 4,wherein said upper and lower switches are MOSFETs.
 8. The circuit asclaimed in claim 5, wherein said upper and lower switches are MOSFETs.